1. Field of the Invention
This invention relates generally to the field of indicating a point of contact between two objects. In a particular embodiment, the invention relates to a ball treated so as to leave a transient visible mark at the point of contact with a surface.
2. Background
A number of games, particularly tennis, are played on a court marked with boundary lines. During playing of the game, it is important to know when a ball lands outside of the boundary lines since this will affect the scoring of the game. In games where the ball is traveling at high speed, it is frequently difficult to visually determine if the ball has landed xe2x80x9cinxe2x80x9d or xe2x80x9coutxe2x80x9d of bounds. Line judges are typically employed in professional matches to make such determination. Their calls are important to the outcome of the game and often incite heated reaction from the players and spectators.
A number of methods and systems have been proposed for automatically determining whether a game ball, particularly a tennis ball, is xe2x80x9cinxe2x80x9d or xe2x80x9coutxe2x80x9d. Many of these require specially prepared courts and/or sophisticated tracking equipment. A system described in U.S. Pat. No. 4,109,911 utilizes a ball with a chemically treated surface that provides a transient indication when it contacts a chemically treated court surface. Thus, even this system requires a specially prepared court.
The present invention provides a game ball with a transient marking feature that may be used on any court surface. No preparation of the court surface is required. The game ball is coated with a temporary marking agent that is encapsulated or otherwise sealed against exposure to the air. When the ball contacts a surface with sufficient velocity, a small quantity of the marking agent is liberated to provide a visible mark on the surface at the point of contact. Similar to a xe2x80x9cdisappearing inkxe2x80x9d, the marking agent dissipates upon exposure to air.